1. Field of the Invention
The present invention relates to a rotational angle sensor, and more particularly to a sensor having a magnetoresistance element for detecting a rotational angle of a rotating shaft.
2. Description of the Related Art
Recently, as to a sensor for detecting a rotational angle or a rotational position, it has been proposed to employ a magnetic sensor for providing a non-contact mechanism or reducing a loss of inertia of a shaft. For this magnetic sensor, used is a magnetoresistance element, a planar surface of which is disposed to face a permanent magnet mounted on the tip end of the shaft, for example.
As the magnetoresistance element, a semiconductor magnetoresistance element and a ferromagnetic magnetoresistance element are known. The former utilizes a property of a semiconductor whose electric resistance varies in a magnetic field. The latter utilizes a property of a ferromagnetic substance whose resistance varies anisotropically depending upon an angle defined by a magnetizing direction and a current direction in a magnetic field. This property of the ferromagnetic substance is called an anisotropic magnetoresistance effect, which is distinguished from a negative magnetoresistance effect in which its ohmic value varies depending upon the magnitude of the magnetic field. Namely, in an ordinal ferromagnetic substance, its ohmic value reaches the maximum when the current direction is parallel with the magnetizing direction, whereas it reaches the minimum when the current direction is perpendicular to the magnetizing direction, according to the anisotropic magnetoresistance effect. In order to make use of this effect, a ferromagnetic thin metal film is deposited on a planar surface of a substrate in a folded line fashion to constitute a ferromagnetic magnetoresistance element. Then, a magnetic sensor which includes the ferromagnetic magnetoresistance element as formed in the above is disposed on either an end face of a shaft or a position opposite to the end face, and a permanent magnet is disposed vis-a-vis, as disclosed in Japanese Patent Laid-open Publication No. 62237302, for instance.
However, in the case where the permanent magnet and the planar surface of the magnetoresistance element are arranged to face each other as disclosed in the above publication, a magnetic field applied on the magnetoresistance element is varied in response to the axial movement of the shaft. Although the ferromagnetic magnetoresistance element may operate in a relatively small magnetic field, this element will not operate, if it is positioned far away from the permanent magnet so that a sufficient magnetic flux is not applied to the element, thus will cause a measuring error. Therefore, the clearance between the planar surface of the magnetoresistance element and the permanent magnet should be maintained to be a certain distance. Accordingly, the shaft must be supported to prevent at least its axial movement, so that the support structure of the shaft is required to be precise and assembled accurately.
When the ferromagnetic magnetoresistance element is driven by a constant-current source or a constant-voltage source, its output is decreased substantially linearly with the increase of ambient temperature, since the ferromagnetic magnetoresistance element has a positive temperature coefficient, which is made smaller in case of using the constant-current source for driving the ferromagnetic magnetoresistance element than in case of using the constant-voltage source. Accordingly, two or four blocks of magnetoresistance elements are interconnected to form a bridge circuit, or other measures are taken, to avoid a fluctuation of the output due to a change in ambient temperature. Thus, in the magnetoresistance element including the above-described ferromagnetic magnetoresistance element, its output characteristic is varied due to the change in ambient temperature. Therefore, the temperature compensation is needed in the case where the ambient temperature, i.e., the externally environmental temperature is varied. In general, various electric circuit measures are provided in order to compensate for the output according to the change in ambient temperature. For instance, in Japanese Patent Laid-open Publication No. 63-42403, proposed is a temperature compensation circuit in which a series circuit of a temperature compensating diode and a resistor is interposed between a constant-current circuit for feeding a constant-current to a bridge of the magnetoresistance element and a constant-voltage circuit for driving the constant-current circuit, and an intermediate point of the series circuit is connected to the constant-current circuit. However, in the above-described temperature compensation circuit, an element different from the magnetoresistance element for detection is added for use of the temperature compensating element, so that both elements have their individual temperature characteristics respectively. Thus, if the element for detection and the element for compensation are different from each other in their temperature characteristics, these elements shall be coordinated each other. However, it is very difficult to stably perform the temperature compensation within the whole temperature range in measurement. Further, the element for compensation is to be selected individually and adjusted according to the change in ambient temperature under various conditions, so that increase of the cost will be caused.
In an internal combustion engine equipped with an electronic controlled fuel injection system, there is disposed a throttle position sensor whose output signal is used for fuel injection control. The throttle position sensor is connected to a throttle valve shaft, and for instance outputs a throttle opening angle signal which varies in response to a throttle valve opening angle (hereinafter simply referred to as a throttle opening) and an idle signal which is an ON or OFF signal depending on a condition of the engine in an idle region or an output region. As disclosed in Japanese Utility Model Laid-open Publication Nos. 59-41708, 59-115304 or 62-81004, it is common that the throttle opening signal is provided as an analogue voltage output proportional to the throttle opening, while the throttle opening signal may be an ON or OFF signal produced in response to the throttle opening. In the throttle position sensor, therefore, there is provided a potentiometer connected to the throttle valve as described in the above publications. Namely, a movable contact slides on a resistor in response to opening operation of the throttle valve to output a voltage between one end of the resistor and the movable contact. In closing operation of the throttle valve, a normally-open fixed contact for producing the idle signal is closed by another movable contact connected to the throttle valve to output an ON signal representing the idle region. Since the potentiometer has been employed in the prior throttle position sensor, the sliding movement between the resistor and the movable contact is necessary, that is the mechanical contact between the resistor and the movable contact is necessary. Accordingly, various measures should be taken to cope with the troubles caused by the mechanical contact such as wear or the like of the movable contact or the resistor. Among those measures, it is useful to employ a rotational angle sensor of a non-contact type for the throttle position sensor. While it is relatively easy for this sensor to detect the throttle opening, it is difficult to detect the idle region. Thus, separate switch means is necessitated, so that employment of the non-contact type is made rather insignificant. For instance, in the case where the magnetic sensor having the magnetoresistance element is employed, it is difficult to provide an accurate threshold level for the idle region, since the change in the output of the magnetic sensor is gentle in a region where the throttle opening, i.e., the rotational angle is small.